Long story short: If Wilhelm is giving you trouble, give Moxxi a lot of tips. That SMG just fucking MURDERS him.

Is it still a randomized gun, I wonder, or a static drop? And if I have the 10K now, should I give it now, or does the weapon scale to my level, meaning I shouldn't give the money until immediately before that fight (or as high level as I can get).

Moxxi only ever gives you two guns. An Acid SMG or a Fire SMG. I guess if you're unlucky, you'll get the fire one. Just exit the game without saving and reload and try again.

It IS set to your level, so the longer you wait, the higher its stats will be, but to be honest, the difference between my level 15 one and level 18 one is fairly minimal. (it's a few points of damage, and a pip here and there.)

If you require help with him, though, just holla. My level 24 siren has that SMG and will quite frankly tear him to shit.